Lined up in uniform (7)
I believe the answer is:
dressed
'lined up' is the definition.
('dress' can be a synonym of 'line up')
'uniform' is the wordplay.
I cannot really understand how this works, but
'uniform' could be 'dress' (dress uniform is a kind of uniform) and 'dress' is located in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for dressed that I've seen before include "bandaged" , "put bandage on" , "in robes?" , "Arranged" , "in fancy packaging" .)